The present invention relates to an ejector which is used with a pneumatic booster for a brake apparatus so that it enhances the magnitude of vacuum established in a vacuum chamber of the booster.
Generally, in a brake apparatus for an automobile, a pneumatic booster is provided so as to increase a braking force. In this pneumatic booster, an intake vacuum of an engine is introduced into a vacuum chamber of the booster, and, due to a differential pressure relative to atmospheric pressure, a thrust force is generated in a power piston provided in the booster, thus increasing a force for operating the brake apparatus.
Usually an engine provides an intake vacuum which is sufficiently high for this purpose. For example, an engine provides an intake vacuum in a range of −300˜−400 mmHg even in its idling condition. However, the intake vacuum may drop to an undesirable level, say to a range of −100˜−200 mm Hg, in some occasions. For example, when a load on the engine is increased by actuating an auxiliary appliance which utilizes the power from the engine, such as an air conditioner and a power steering motor, a throttle is opened wider in order to increase the engine power with the rate of rotation of the engine being unchanged. The throttle thus decreases the differential pressure across it, thereby lowering the intake vacuum. This tendency is especially eminent in case of small automobiles.
Furthermore, during the time immediately after a cold start of an engine, a sufficiently high negative pressure cannot be obtained either. In these cases, decreased vacuum in the vacuum chamber lowers a servo power provided by the power piston of the booster.
Therefore, proposals have been made to employ a pneumatic booster utilizing an ejector, so as to increase a negative pressure introduced into the vacuum chamber.
Although the use of ejectors for enhancing vacuum in a vacuum chamber of a pneumatic booster is known, it has been always desired to make an ejector more efficient so that it can provide a desired high vacuum even from a lower operation vacuum created by an engine. In other words, it has been desired to improve an ejector so that a range of low operational vacuum pressures which has not been useful to establish a desired high vacuum in the vacuum chamber of a booster becomes usable.
Furthermore, it has always been desired to improve an ejector so that it provides a greater flow rate in evacuating the vacuum chamber so that the desired high vacuum can be resumed in the vacuum chamber in a short time. This is very important because, when a brake is once used, it must prepare for the next use in a very short time by establishing the desired high vacuum in the vacuum chamber.